MATLAB: Dynamically name a struct

dynamic variablefield namesstructstructsvariable

I'm generating a GUI which users will type the name of a test and then generate structs based off of the test type. Right now they have 4 selections to choose from for struct naming (Body, Head, Leg, and New). I need this ability for the "New" element so that this code doesn't become useless when new test types are generated. (I'm only stating this because there are plenty of answers with naming struct fields that argue that I should never need to do this).

I have a temporary variable k, that holds the struct until the end, where I would like to essentially go (variable) = k, and have it generate

k.TestName k.SignalName k.Signals.. etc

(variable) = k;

(variable).TestName (variable).SignalName (variable).Signals.. etc

Any and all ideas would be greatly appreciated.

Best, ML

Best Answer

While it is possible to generate variables dynamically (just use eval) it's not recommended practice. Generating the variable name from user input, particularly, is asking for trouble. The way variables work in matlab, they shadow functions of the same name. So if the user decides to name your variable plus, that's addition broken in the rest of your program, name it eq and it's comparison that's broken.

You also have no guarantee that the string entered by the user is a valid variable name. So you have to pass the user input through genvarname or matlab.lang.makeValidName, but then the actual variable name may be different from the user input. Possible confusion...

I'm not clear on your end goal, but if what you want is a mapping between an arbitrary (user input) string and a value then you could use a map. Matlab's implementation of maps is a bit clunky but it does work.

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MATLAB: Matlab change structure field name by comparing it with a list of conversions

Mustahsan - to be clear, you have a struct with a number of fields. If any of those field names appear in column 1 of your labellist.mat file, then you want to replace that field name with that in column 2. Is that is correct, then you can try the following. Suppose you have a cell array of field names and their replacements (read from your file) as

 namesArray = {'fieldA' 'fieldANew' ; 'fieldB' 'fieldBNew' ; 'fieldC' 'fieldCNew'};

and a struct like

 myStruct = struct('fieldA',0,'fieldB',1,'fieldD',42);

where

 myStruct = 
    fieldA: 0
    fieldB: 1
    fieldD: 42

We can then create a list of the fields in the structure, iterate over each one and replace where necessary

 structFields = fields(myStruct);
 for k=1:length(structFields)
     fieldName = structFields{k};
     % find the index of this field name in the first column of namesArray
     idx = find(strcmp(namesArray(:,1),fieldName));
     if ~isempty(idx)
         % field name exists so get the replacement
         newFieldName = namesArray{idx,2};
         % create the new field
         myStruct.(newFieldName) = myStruct.(fieldName);
         % remove the old field name
         myStruct = rmfield(myStruct,fieldName);
      end
 end

And the field names in myStruct are replaced

 myStruct = 
       fieldD: 42
    fieldANew: 0
    fieldBNew: 1

MATLAB: Does isfield return with false when the field does exist in a structure

Avoid creating dynamically named variables using eval in MATLAB. This is poor practice as has been explained many times on this forum, and is not recommended by MATLAB themselves:

http://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

When you are a beginner it seems like a cunning and fast way to store information, but actually it is really bad practice to name your variables dynamically. MATLAB is also not intended for this kind of variable naming: if you continue to include data in the variable names then you will find yourself fighting many more of these battles against MATLAB.

However when you use more appropriate storage for your data (and meta-data) then you will suddenly find lots of MATLAB functions that do many useful operations for you, quickly and easily.

In your case a much more robust solution would be to use structures , where you can include fields for each kind of data (e.g. Process type, Flow data, Temperature data, Notes, Units, etc), or cell arrays . There are many functions that support working on structures and cell arrays, and can access these data easily, and they can also be used in vectorized code (which is something you need to learn about). Structures can be non-scalar , and you can even define structure fieldnames dynamically.

Placing your data in a structure or cell array also makes it much easier to pass to functions: can you imagine the fight you would have trying to pass hundreds of dynamically named variables to a function?

If you have a newer version of matlab you can also use a table , which stores the data together in one array but also allows key-name access to the columns. This might be a good alternative for your data.

In case you are interested, here are some pages explaining why dynamically assigning variable names is a really bad idea in MATLAB:

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

http://www.mathworks.com/matlabcentral/answers/56124-eval-is-evil-using-variables-created-dynamically-info-retrieval

http://www.mathworks.com/matlabcentral/answers/51946-systematic-do-not-use-global-don-t-use-eval

http://www.mathworks.com/matlabcentral/answers/79281-create-a-structure-name-based-on-the-string-of-a-variable-create-fieldnames-in-this-structure

http://www.mathworks.com/matlabcentral/answers/74778-loop-file-and-variablenames-when-importing

Best Answer

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